Photodetectors are used to detect light of a given wavelength and produce a current proportional to the intensity of the detected light. A photodetector may be supplied with a bias voltage. The output of the photodetector may vary with variations of the bias voltage.
A prior art photodetector circuit 100 is shown in FIG. 1. The circuit includes a photodetector 150 between a high voltage supply 110 and a transimpedance amplifier 170 (TIA) including a voltage amplifier 140 and a feedback resistor 130. The signal (photocurrent) from the photodetector 150 is amplified by the transimpedance amplifier 170, thereby converting a photocurrent from the detector into a voltage 160 suitable for post processing, for example, by a post processing circuit (not shown).
Any variation of the high voltage supply 110 is coupled to the input of the transimpedance amplifier 170 through the impedance of the photodetector 150. Examples of such variations include noise from power supply or intentional high voltage supply variation for Gain control in the case of an avalanche photodiode (APD) or a silicon photomultiplier (SiPM). As such, any voltage variation from the high voltage supply 110 is converted into a current which is amplified by the transimpedance amplifier 170 and added to the signal coming from the photodetector 150. The voltage resulting from the variation of the high voltage supply 110 may impact the output signal 160 at the output of the transimpedance amplifier 170 in different ways. For example, the variation may be seen as noise impacting the detection capability of a post processing system (not shown) processing the optical signal. Similarly, the variation may create an offset at the output of the transimpedance amplifier 170. In addition, the variation may impact the frequency response of the receiver 100 when voltage from the high voltage supply 110 is correlated with photocurrent.
A second prior art circuit 200, as shown in FIG. 2 and described by US Patent application 2004/0130397 A1 and U.S. Pat. No. 7,561,812, B2, attempts to provide immunity to noise from a voltage source 210 with a differential transimpedance amplifier 270. The second prior art circuit 200 connects a photodiode 250 between the voltage source 210 and an inverted input to the differential transimpedance amplifier 270, while placing a capacitor 220 between the voltage source 210 and a non-inverted input to the differential transimpedance amplifier 270. Capacitor 220 has a value equal to the junction capacitance of the photodetector 250. Resistances 230 and 235 are respectively placed across the inverted and non-inverted sides of the amplifier 240. US Patent application 2004/0130397 A1 and U.S. Pat. No. 7,561,812, B2 both target low voltage applications and offer limited performance for high frequency applications using avalanche photodiodes (APD) and not sufficient for a silicon photomultiplier (SiPM).
For high frequency applications using Avalanche Photodiodes or SiPM, a simple capacitance may not accurately model the photodetector and high voltage required to bias the APD or SiPM, limiting the usefulness of this prior art.
Furthermore, both circuits are limited to common mode current (the current injected simultaneously at the inputs of the differential transimpedance amplifier) within the dynamic range of each input of the differential transimpedance amplifier. A large variation in the voltage source 210 can saturate the transimpedance amplifier 270.
Variations of a high voltage supply are addressed by U.S. Pat. No. 5,696,657, where the rate at which the high voltage supply is varied is carefully controlled in order to minimize the amount of current flowing through the APD. This is an important limitation of a system if it is desirable to change the APD gain quickly as it is the case in range finding application where a close target may reflect a significant amount of light back into the receiver thus saturating the receiver and blinding the receiver from detection of targets located in the line of sight at a greater range. Prior art also discloses various ways of managing high current by extending the dynamic range of the transimpedance amplifier.
The pseudo differential transimpedance amplifiers discussed in U.S. Pat. No. 6,803,825 B2 and U.S. Pat. No. 6,784,750 B2 sense the high voltage supply variation to give an indication of the photocurrent through the use of a coupling capacitor on the high voltage. This configuration uses the coupling capacitor as a way to measure the AC photocurrent regardless of the source of the AC current flowing through the photodetector. These circuits assume that the AC current comes only from the photocurrent as they cannot react to large voltage supply variation. Also, these circuits inject a DC current at the input of the transimpedance amplifier which cannot be used for fast signal variation. Therefore, there is a need in the industry to address the above shortcomings.